Expansion type envelopes are generally constructed in one of two forms, i.e., with a single center seam or with paired side seams. Moreover, expansion type envelopes are also generally constructed with gusseted side walls which provide the expansion characteristics of the envelope. In such cases, the envelope blank requires several forward and reverse folds to form the gussets and thus is difficult to manufacture on automatic equipment.
The present invention comprises an expansion envelope of the side seam type but without gussets. Accordingly, the envelope of the present invention may be readily and easily formed either by hand or on automatic equipment. The envelope is formed by applying adhesive to one or both of the side seam flaps, the flaps are then folded inward onto the front and back panels, and the front and back panels are finally folded into overlying relation to adhere the side seam flaps together.
U.S. Pat. No. 3,552,640 discloses a typical prior art construction for an expansible gusset envelope with side flap seams. In this patent, the gusset panels must be prefolded before the side seams are formed. Meanwhile, U.S. Pat. No. 3,860,164 discloses a modified construction for a gusseted expansible envelope wherein the glue flaps may be reversely folded before seaming. And, finally, U.S. Pat. No. 3,955,749 discloses another side seam gusseted expansion envelope wherein the adhered portions of the side seams project into the envelope after it is formed and set up for use. Accordingly, it may be seen that the expansible envelope of the present invention is of a much simplified design than the conventional and modified gusset designs shown in the prior art.